This invention relates to a supersonic wave apparatus installed in a vehicle for detecting a vehicle height relative to a road surface and the like with a reflected wave in a supersonic wave sensor. More particularly, this invention relates to an apparatus for preventing a turning-in or "roundabout" of the wave in the supersonic wave transmitter and receiver for the vehicle.
As described in Japanese Patent Laid-Open No. 116959/75, various types of supersonic wave sensors have been developed and used for detecting a distance to an object and a presence or absence of the object with a reflected wave from a supersonic wave sensor. Further, various types of supersonic wave sensors have been developed to be installed in the vehicle when high performance is required. In particular, such a sensor may be installed at the outside of the vehicle, for example, to act as a vehicle retracting sensor or a so-called "back-sonar" used for detecting an obstacle placed behind the vehicle. A supersonic road-surface sensor may also be installed in a so-called supersonic suspension which has been developed for measuring a variation in vehicle height corresponding to a road surface condition, such as a bad road surface, during running of the vehicle and controlling an attenuation force of a shock absorber to the most appropriate form. Thereby, a comfortable feeling in riding and a stability in driving the vehicle are improved.
FIG. 5 illustrates a side elevational view of a conventional supersonic wave sensor. In FIG. 5, a sender 1 is provided to transmit a supersonic wave to a receiver 2 to receive the supersonic wave from the sender 1. An insulator 3, made of foamed rubber and the like, protects the sender 1 and the receiver 2. A base plate 4 is provided with circuit elements 5 which are protected by a filler agent 6. A case 7 is provided for the foregoing elements In the case 7, a cone-shaped horn part 8 is formed at the front surfaces of the sender 1 and the receiver 2, and a heater 9 is held by a bracket 10 at the surface where the horn part is open to the exterior.
In the above-described prior art, the supersonic wave transmitted from the sender 1 reaches the receiver 2 as a direct wave in the air by diffraction or sidelobe and the like. It has been found that a wave is also transmitted along another path through a clearance provided by an assembly of the case 7, the heater 9, and the bracket 10.
The phenomenon in which the supersonic wave is transmitted directly from the sender 1 to the receiver 2 is a so-called "turning-in" or "roundabout" phenomenon. It has thus been necessary to provide a countermeasure to electrically prevent the circuit from sensing the received signal caused by a turning-in for a period from a time t.sub.o (m-sec), from transmitting of the supersonic wave, to a given time T.sub.1 (m-sec) as shown in FIGS. 6 and 7 in order to prevent any erroneous operation caused by the turning-in of the wave. The characteristic curve i shown in FIG. 6 denotes a characteristic of the received signal caused by the turning-in of the wave. The characteristic curve j also shows a characteristic of the received signal of the reflected wave accepted from the object to be sensed by the supersonic wave sensor, for example, from a road surface and the like.
FIG. 7 shows a characteristic waveform of a timing pulse for use in exciting the sender 1 of FIG. 5. Therefore, as the duration time of the characteristic curve i of the turning-in action is extended, it is necessary to set an elongated prohibitive time T.sub.1 (m-sec) Thus, an object at a short distance, wherein the time taken by the reflected wave to reach the receiver 2 is short, may not be detected. This resulting omission becomes a significant disadvantage in the supersonic wave road surface sensor used for detecting the road surface at a short distance. Thus, in the supersonic wave sensor, such as a supersonic wave road surface sensor, used for detecting an object at a short distance, either shortening the duration time of the turning-in phenomenon or reducing the frequency of the turning-in phenomenon has been a continuing problem.